I admit that I might have some void in my memory, but I do remember part of this pretty well.  Because CS had the 11/40E with writable microcode (they had been developed for C.mmp). The CS PDP-11's running UNIX (IUS and SUS) had CSV/CRET instructions (as well as a couple of others for BLISS and support capabilities, IIRC). Someone (Paul Rubin I think), had modified the original V6 compiler to generate CSV/CRET.  That meant that binaries from CS would not run on the 11/34's which most of the UNIX box on campus had by then become.  Ted and I were in EE and there was a small battle as to which systems were more current.  Since Ted worked officially for USG, he had a direct line to much of there stuff.    Ted had xerographic proofs of the book when it was being reviewed BTW.   When CMU got the new Ritchie compiler and I had the task of rippling it out to all the sites in EE, Mellon Institute, et al.  CS did not pick it up originally because it did not have the CSAV/CRET changes.   CMU did not get V7 until late winter '78/early spring '79 around the time I was leaving.  

So, we definitely had a Ritchie based K&R compiler with stdio.  I had thought that come to us with Typesetter C, but it sounds like it had a different path.   FWIW: that would actually be the compiler sources I started with at Tektronix 6 months later, when I started to retarget it for what would become the 68000 (it was not yet released and was an experimental chip), as I had brought the CMU UNIX systems with me.